The present invention relates to thick transparent articles obtained from polyalkylene terephthalate resins having characteristics suitable for the preparation of the said articles.
Thick transparent sheets obtained from amorphous thermoplastic polymers such as PVC, polymethylmethacrylate, polycarbonate and polystyrene are known.
Such sheets have various disadvantages such as, for example, the presence of plasticizers in PVC, which tend to migrate to the surface or the difficulty recycling PMMA and PC sheets, and the inflammability and fragility of PMMA sheets.
Relatively thin transparent sheets obtained from crystallizable modified PET having a cold crystallization temperature between 120.degree.-160.degree. C. are known.
It is known from U.S. Pat. No. 2,965,613 that copolyethylene terephthalate resins containing 5-15% isophthalic acid units having an intrinsic viscosity of 0.5-0.7 dl/g (obtained using known methods by esterification/polycondensation of mixtures of terephthalic and isophthalic acid or by transesterification with ethylene glycol of mixtures of terephthalate and dimethyl isophthalate and polycondensation of the glycol esters obtained) are crystallizable although at very much lower rate than that of polyethylene terephthalate homopolymer. Only when the unit content of isophthalic acid is very high (25-85%) are the resins no longer crystallizable. In this case Tg is very low and makes the resin unsuitable for the preparation of products having sufficiently good mechanical properties.
In U.S. Pat. No. 4,234,708 copolyethylene terephthalate/isophthalate copolymers containing 5-15% of units deriving from isophthalic acid are described having very high intrinsic viscosity and melt strength, suitable for the preparation by blow-extrusion of thin wall transparent containers.
The copolymers are prepared by polycondensation of mixtures of terephthalic and isophthalic acid and ethylene glycol or by transesterification of mixtures of dimethyl terephthalate and isophthalate and subsequent polycondensation of the esters, in the presence of a branching agent such as, for example, trimethylolpropane and pentaerythitol and a chain terminator (benzoic acid and the like).
The intrinsic viscosity of the resin that is obtained (which is less than 0.7 dl/g) is brought to values greater than 1 dl/g by solid state polycondensation (SSP).
After SSP, the resins have a rather higher viscosity in the molten state (10.sup.4 -10.sup.5 Pa.multidot.s at temperatures from 265.degree. C. to 300.degree. C. in the absence of shear forces) which falls drastically, under the action of shear forces, to values of 10.sup.2 -10.sup.3 Pa.multidot.s.
Such a high sensitivity to shear forces, although on the one hand suitable for blow moulding operations, is not suitable for the preparation by extrusion of thick products where it is necessary that the melt maintains a sufficiently high viscosity even under the action of shear forces.
Heat formed articles having a thickness of 1-20 mm obtained from sheets of crystallizable polyalkylene terephlthalate having a crystallization temperature of between 120-160.degree. C. are known from WO-A-97/12750.
In the examples, there are heat formed sheets with the thickness of 5 mm at most.
The possibility of preparing thick transparent articles and sheets having good mechanical properties starting from polyalkylene terephthalate resins has been considered, but the problem until now has not been resolved.